Method of producing phosphate coated ferrous articles



UNlTED, STATES PATENT OFFICE METHOD OF PRODUCING PHOSPHATE COATED FERROUS ARTICLES Gerald G. Bomig, Elldns Park, Pa., assignor to American Chemical Paint Company, Ambler, Pa., a corporation of Delaware No Drawing. Application May 12, 1938, Serial No. 19,276

18 Claims. (01. 204-1) This invention relates to the art of coating the character described is kept below certain iron or steel and is particularly concerned with a limits for the purpose of increasing its smoothprocess for providing iron or steel with an adness and resistance to corrosion. herent non-metallic coating. More specifically As in the former application, the iron or steel 5 it is directed to an electroplating process the to be coated is used as an electrode in an electroprincipal objects of which are to provide an iron lyte containing phosphate anions and the cations or steel article with a coating which will protect of the metal phosphate desired in the coating and it from corrosion, to ensure an enduring bond through which electrolyte a reversing current is between the metal and an applied finish, to propassed by means of said electrode.

30 vide steel or iron with a stainless coating, and, Almost any kind of an electric current may be in general, to greatly improve the art of coating employed provided, however, that it changes its iron and steel so as to produce a finished article direction at least once during the coating operahaving a maximum degree of protection in a tion. Currents that fluctuate in direction, even minimum of time and at a'minimum expense. though not periodically, will produce a coating A further object is to provide a coating for in accordance with my invention although, from 15 iron or steel which will be a comparatively poor 9, commercial standpoint, I prefer the alternating conductor of electricity when in a dry condition current oi standard frequency (say from 25 to 60 but which, when wet, will have suiiicient concycles per second). Nevertheless I do not wish to ductivity to enable the article to be electroplated be limited to any particular frequency since greatfrom alkaline electrolytes with copper and other er or less frequencies may be used with consld' 20 metals. More particularly it is an object of my erable success. The wave shape or the duration invention to provide iron or steel with a waterof the pulses of the current are not material so insoluble coating containing phosphate which long as sumcient effective ampere-minutes are will be free from discoloration or stains and which provided will have a very great ability to retain an ap- In connection with the present disclosure I 25 plied finish, such as paint, varnish, lacquer, preferacurrent density which is in the neighbor- Japan, or the like. hood of 35 amperes per square foot of metal to be Before proceeding further with the present coated, although this may be altered over a wide disclosure it is desired to point out that this range without changing the nature of the coating,

application is a continuation in part of my apthe necessary time of treatment alone being af- 30 pllcation Serial No. 515%, filed January 4, 1936, fectecl. and that the invention disclosed herein is in the The cations of the electrolyte may be one or nature of an improvement upon some phases of more of the metals in that class comprising zinc, the invention disclosed in my earlier copendlng cadmium, calcium, nickel, cobalt and manganese.

application Serial No. 701,869, filed December 11, Cations of the alkali metals may be present with- 35 1933. In this earlier application No. H.869 the I out doing harm, but they cannot replace cations broader aspects of the invention are disclosed and of the above metals in the process. claimed and it is pointed out that the process The essential anions of the electrolyte are phostherein set forth is applicable to a rather large phate anions, but the presence therein of other class of metals which are less noble than lead anions such as sulphate, nitrate, nitrite, chloride, 40

and which are capable of forming water insoluble arsenate, etc., may be tolerated without substanphosphates such, for-example, as iron, steel, zinc, tial harm, unless these are present in excessive cadmium, brass, nickel, chromium, tin and mag amounts. nesium. The removal of iron from the electrolyte, re-

I have now found that in the case of iron or ferred to above. may be accomplished by any 45 steel it is possible to improve the coating both suitable chemical means. I prefer, however, to as to smoothness of texture and resistance tov precipitate the iron as ferric phosphate. This is corrosion by reducing its iron content as much accomplished by adding to the electrolyte, in as possible, since the presence of more than a which the iron is present largely as ferrous iron,

certain percentage of iron has a deleterious efa suitable oxidizing agent to convert the ferrous 50 feet on the resistance of the coating to staining iron into ferric iron. 'i'he low solubility of ferand to its ultimate failure under the influence of ric phosphate in solutions of the low acidity of atmospheric corrosive agencies. the electrolyte causes the iron to be precipitated,

The present disclosure, therefore, relates to a and the small amount of ferric iron remaining in means whereby the iron content n. a coat g o; solution, usually 01' the order of 132% by weight, as

is insuflicient to interfere with the production of the improved coatings of the present invention. The best oxidizing agent which I have discovered for this purpose and one which functions also as an excellent addition agent, as will be explained below, is nitrous acid which may be added as such or in the form of one of its salts, from which latter the acid electrolyte immediately liberates nitrous acid.

By way of specific example, I will now describe my improved process as it may be applied to the coating of a piece of cold rolled sheet steel as, for instance, an automobile fender or other stamping.

The stamping is first freed from extraneous matter. The manner in which this is accomplished depends, of course. on the nature of the extraneous matter. This, in itself, forms no part of the present invention, although it might be said that scale and rust may be removed with an acid pickle followed by washing with water; oil and grease may be removed by washing with a suitable solvent or by means of an alkaline cleaning solution; and paint may be removed by boiling in a bath of caustic soda or other alkali, and then washing with water.

An electrolyte is then provided, using materials in the following proportions:

Formula No. 1

Zinc oxide pound 0.120 75% orthophosphoric acid gallon 0.040 Sodium nitrite pound 0. 0034 Water, to make gallon 1. 000

The zinc oxide is mixed with 0.25 gallon of the water, then while stirring, the acid is slowly added and the stirring continued until the admixture becomes clear indicating that the oxide is completely dissolved. This forms a concentrated solution which may be stored until required for use. when the remainder of the water is added to form the electrolyte. The sodium nitrite is not added until just before the electrolyte is to be used since it is not stable in acid solutions. In place of sodium nitrite an equivalent quantity of a solution of freshly prepared nitrous acid may be used, or any other convenient nitrite such as calcium, barium, or cerium nitrite, each in equivalent quantity.

For reasons which will appear more fully hereinbelow, I prefer to employ a steel vat of a size suitable for the work in hand. Sufllcient of the foregoing electrolyte is placed in the vat to compietely submerge the stamping, which is connected to one terminal of a 60 cycle alternating electric circuit, the other terminal being connected to the steel vat. The circuit is then completed and the current adjusted to a density of approximately 35 amperes for every square foot of stamping surface exposed to the electrolyte.

At first there is a rapid evolution of gas from the stamping, but this gradually subsides'and reaches a minimum in from two to four minutes. During this time a coating gradually forms on the stamping and reaches a maximum thickness at about the time that the gasing reaches a minimum. The stamping is then removed from the ing, such, for example, as steam coils. In this connection it should be pointed out that the electrical resistance is less at higher temperatures, which, of course, is an advantage sinceit permits the use of a lower voltage to supply a given current. However, too high a temperature is to be avoided as it makes for excessive evaporation. Moreover, excessive heating has another disadvantage which will be further described below.

The time required to produce the coating depends principally on the current density employed, and on the kind and surface condition of the steel being coated. It is affected in minor degree by such factors as the concentration of the electrolyte, the ions associated with the phosphate ions, and the temperature of the electrolyte.

So long as the electrolyte contains the propercations and anions, the composition, concentration, temperature and current density are not critical. The completion of the coating generally requires from 70 to ampere minutes, effective. per square foot of surface coated. In practice very satisfactory results are obtained by adjusting the factors so that the coating is completed in about four minutes.

When the coating has completely formed, continuing the electrolyzing does not appreciably increase its thickness, nor change its composition nor electrical resistance: hence metal that has been coated by the process may be used as a permanent electrode to co-act with a succession of pieces of metal to be coated. This being the case, a steel vat may be used for holding the electrolyte and may at the same time serve as an electrode.

In the practical operation of the process on a succession of stampings, the maintenance of the electrolyte in proper working condition is essential. The principles of this maintenance are simple. In what follows the active cations of the electrolyte are spoken of as zinc cations although the same considerations apply to solutions containing any of the cations of the class described with which the process is operable.

First of all, the concentration of iron in the electrolyte must be kept at a low level, preferably below 0.25% by weight, and never above 0.6% by weight. Secondly, in order to obtain a coating of maximum smoothness, a small concentration of acathodic depolarizing or addition agent must be maintained. Nitrates or nitrous acid are very suitable addition agents. Nitrates may be tolerated in fairly large amounts, but nitrous acid should not be present in amounts greater than about 0.03 pound per gallon of the electrolyte. For reasons which will appear below I prefer to use nitrous acid or one of its salts for this purpose. Thirdly, the proper concentrations of.zinc ions and phosphate ions must be maintained.

With respect to the first of these, namely, the content of iron in the electrolyte, it should be understood that during the coating of iron or steel articles more or less iron goes into solution in the electrolyte, and. as stated above, I prevent its accumulation by the use of a suitable precipitating agent which will oxidize the ferrous iron into ferric iron, the ferric phosphate thereby resulting being only slightly soluble in the electrolyte so that most of it is thrown down as a precipitate which can be removed from time to time.

As indicated above, I prefer to use nitrous acid or one of its salts, although any other suitable oxidizing agent may be employed which will convert ferrous to ferric iron with reasonable arouse rapidity in the weakly acid electrolyte with which the process isoperable. Incidentally nitrates will not perform this function, but other examples of oxidizing agents which will successfully cause the desired precipitation are permanganates, dichromates and hydrogen peroxide, in addition to nitrous acid or its salts mentioned above.

However, it is to be understood that while the removal of ferrous iron is conveniently accomplished by the use of a suitable oxidizing agent, I do not desire to be limited to that means of I removal of ferrous iron. Any suitable chemical means for this removal which does not markedly change the composition of the electrolyte in other respects is suitable. As an example of such means the addition of a sufllcient quantity of a water soluble ferricyanide such as sodium ferricyanide, will cause the ferrous iron to be precipitated as the insoluble ferrous ferricyanide known as Turnbulls Blue.

The maintenance of the proper concentrations of phosphate and zinc ions .is best accomplished by additions of zinc phosphate solutions, such as the concentrated solution described in connection with the foregoing formula.

The reason why I prefer to use nitrous acid or one of; its salts is because the excess of this material which has not been consumed in oxidizing ferrous iron acquired by the solution will also function as a very suitable depolarizer or addition agent, one of the functions of which is to promote smoothness in the resulting coating. 1

should like to point out, however, that other addition agents may be employed if desired, such, for example, as a. nitrate of the alkali metals, magnesium, cerium and zinc any of which, I have found, willpromote smoothness in the resulting coating. Such other addition agents, however, can not be employed as precipitating agents for the iron and if used some suitable precipitating agent must still be resorted to in accordance with the instructions already given.

I have also discovered that smootheness of coating is further enhanced if the iron or steel to be coated is slightly rubbed as by wiping with a cloth or lightly brushing before immersing it in the electrolyte. The reason for this effect is not known, but it is an experimental fact that work so wiped or brushed acquires a smoother coating than where not so rubbed. Moreover, I have found that less iron goes into solution from the surfaces being treated if. they are first rubbed so that the rubbing results in an economy in the use of oxidizing agent for removing the iron.

It should also be noted that a high temperature of the electrolyte likewise promotes the acquisition of iron by the solution from the work so that the selection of the temperature of opera;-

tion involves a balance of economic factors such; as the price of electric power, the cost of oxidizirig' agent, etc.

If, for some purposes, the finished dry coating produced by Formula No. 1 has too high an electrical resistance, a coating of greater conductivity may be produced by using an electrolyte made in accordance with the following formula:

Formula No. 2

Cadmium hydroxide pound 0.338 75% orthophosphoric acid "gallon" 0.050 Water, to make gallon 1.000

To the above is added, just before use, sufficient of a freshly prepared solution of nitrous acid to give a concentration of nitrous acid in the finished solution not over 0.03 pound per gallon.

Such a nitrous acid solution is easily prepared just before it is required by adding to a 2 to 5% solution of calcium nitrite, for example, the calculated quantity of sulphuric acid lust suflicient to liberate the nitrous acid. Such solutions are not stable and must be added to the electrolyte shortly after preparation, say within one-half hour of their preparation I The same nitrous acid so one may be used, during the coating of a succession of pieces of work, to cause the precipitation of the accumulated iron in the electrolyte, the quantity added being so regulated that in addition to the quan-- tity used to oxidize the iron and to replace the small amount lost by normal decomposition in the warm acid solution, a slightexcess is employed; the total quantity added at any time, however, must not" cause the concentration of nitrous acid in the solution to exceed .03 lb. per gallon. A concentration greater than .03 lb. per gallon of electrolyte will markedly decrease the corrosion resistance of the resulting coating.

Various formulas for the electrolyte may be substituted for those already given and still produce satisfactory results without departing from the invention. In the formulas which follow it is to be understood that to each is to be added before use, a suitable quantity of a depolarizer or addition agent as described, and that during the operation of the process the correct quantities of oxidizing agent for removal of the iron are to be added as required.

Formula N0. 3

Zinc sulphate "pound" 0.266 75% phosphoric acid gallon 0.002 Water, tomake gallon 1.000

Formula No.4

Cadmium pound 0.103 Zinc pound 0.060 75% phosphoric acid ....gallon.. 0.100 Water, to make gallon 1.000

Any convenient sources of the cation in the electrolyte may be substituted for those given in the formulas such as free metal, oxide, hydroxide, carbonate or any salt whose anion may be tolerated in the electrolyte and the same considerations apply to the anions of, the electrolytes.

The process produces an integral, water-insoluble phosphate coating which is light gray in color, very finely crystalline, exceptionally smooth and adherent and extremely non-porous in character. Its composition varies, however, according to the composition of the electrolyte used and the nature of the metal coated. It may be said that ingeneral the coatings produced contain a considerably smaller proportion of iron than coatings produced on the same metal by other processes such as the well-known process of Coslett or any of its later modifications. Coatings produced by the process of this application in electrolytes containing substantially no ferrous iron contain relatively low quantities of iron. One such coating produced on a steel stamping by this process, using the electrolyte of Formula No. 1, contained only of iron and more than 38% of zinc, and I have found that the invention makes it possible almost uniformly to hold the content of iron in the coating to not over approximately course, will vary slightly with difierent formulas andpossibly also with slight variations in technique, but for all practical purposes I have found This, of

that the invention makes it possible to keep the iron content generally well below 15%.

After the metal has been coated according to this invention and then washed'and dried, it is in condition to receive an applied organic finish such as oil, paint, lacquer, synthetic enamel, japan or varnish, which may be applied by dipping, brushing or spraying. This may be allowed to air dry or it may be dried in an oven provided its temperature is not high enough to injure the organic finish. Repeated tests indicate that the organic finish holds better to the coating of this invention than it does to the bare metal and remains entirely free from the development of defects usually met with when the finish is applied directly to the metal, hence the invention is useful in preparing the metal for painting, etc.

As the chemicals used in preparing the electrolyte are inexpensive and commercially available, as the amount of electrical energy consumed is relatively small and, as the coatings produced are superior to those of, the prior art,

the process affords a maximum protection' against corrosion at'a minimum cost and constitutes a unique advance in the art of protecting metal against corrosion.

I claim:

1. In the art of coating iron or steel with a water insoluble metallic phosphate, the method which comprises electrodepositing the coating by using the metal as an electrode in an electrolyte containing phosphate anions and the cations of a metal from the class consisting of zinc, cadmium, calcium, nickel, cobalt and manganese, passing a reversing current through said electrolyte by means of said electrode and periodically adding to the electrolyte an agent for the removal ot iron from the solution, said agent being from the class consisting of ferricyanides, permanganates, dichromates, hydrogen peroxide, nitrous acid and a salt of the latter.

2. The method of claim 1 wherein the current is a periodically alternating current.

3. The method of claim 1 wherein the agent for removing iron is added as required to maintain the concentration of iron in the electrolyte preferably below approximately 25% by weight but never over .6% by weight.

4. In the art of coating iron or steel with a water insoluble metallic phosphate, the method which comprises electrodepositing the coating by using the metal as an electrode in an electrolyte containing phosphate anions and the cations oi a metal from the class consisting of zinc, cadmium, calcium, nickel, cobalt and manganese and-an addition agent selected from the class which consists of nitrous acid, salts thereof, and nitrate of the alkali metals, magnesium, cerium and zinc; passing a reversing current through said electrolyte by means of said electrode: and periodically adding to the electrolyte an agent for the removal of iron from the solution, said agent being from the class consisting of ferricyanide's, permanganates, dichromates, hydrogen peroxide, nitrous acid and a salt of the latter in amounts sufllcient to maintain the concentration ferrous iron in the electrolyte preferably below approximately 25% by weight but never over 5% by weight.

5. The method of claim iwherein the current is a periodically alternating current.

8. In the art of coating iron or steel with a water insoluble metallic phosphate, the method which comprises electrodepositing the coating by using the metal as an electrode in an electrolyte containing phosphate anions and the cations of a metal from the class consisting of zinc, cadmium, calcium, nickel, cobalt and manganese,

passing a reversing current through said electro-,

lyte by means of said electrode, and periodically adding to the electrolyte nitrous acid or a salt thereof suflicient to maintain the bath substantially free of ferrous iron.

7. The process of claim 8 wherein the quantity of nitrous acid or a salt thereof is slightly in excess of the amount required.to maintain the bath substantially free of ferrous iron, said excess, however, not to exceed the equivalent of approximately .03 pound of nitrous acid per gallon of electrolyte.

8. The process of claim 6 wherein the current is a periodically alternating current.

9. The process of claim 6 wherein the quantity of nitrous acid or a salt thereof is slightly in excess of the amount required to maintain the bath substantially free of ferrous iron, said excess, however, not to exceed the equivalent of approximately .03 pound of nitrous acid per gallon of electrolyte and wherein the current is a periodically alternating current.

10. The method of depositing a metallic phosphate on an iron or a steel article which comprises electrodepositing the coating by using the article as an electrode in an electrolyte containing the following in approximately the proportions indicated:

Zinc oxide. pound 0.120 75% orthophosphoric acid gallon 0.040 Sodium nitrite pound 0.0034 Water. to make "gallon" 1.000

passing a reversing current through said electrolyte by means oi said electrode, the electrolyte being employed at a temperature in the neighborhood'of about 120 F. and the current density being in the neighborhood of 35 amperes for every square foot of said electrode surface exposed to the electrolyte, and periodically adding to the electrolyte nitrous acid or a salt thereof in an amount not to exceed the equivalentof approximately .03 lb. of nitrous acid per gallon of electrolyte over and above the amount which is necessary to maintain the electrolyte substantially free of ferrous iron.

11. The method of depositing a metallic phosphate on an iron or a steel article which comprises electrodepositing the coating by using the article as an electrode in an electrolyte containlng the following in approximately the proportions indicated:

Cadmium hydroxide pound 0. 33B 75% orthophosphorlc acid pound 0.050 Water, to make gallon 1.000

passing a reversing current through said elecof electrolyte over and above the amount which is necessary to maintain the electrolyte substantially free of ferrous iron.

12. The method of claim 1 wherein the agent for removing iron is added as required to maintain the concentration of iron in the electrolyte preferably below approximately 25% by weight but never over 116% by weight and wherein the electrode is rubbed beiore it is immersed in the 5 electrolyte.

13. The method of claim 6 wherein thequantity of nitrous acid or a salt thereof is maintained slightly in excess oi the amount required to maintain the bath substantially free of ferrous iron, said excess, however, not to exceed the equivalent of approximately .03 lb. of nitrous acid per gallon of electrolyte and wherein the electrode is rubbed before it is immersed in the electrolyte.

5 14. The method oi claim 6 wherein the current is a periodically alternating current and the quantity of nitrous acid or a salt thereof is maintained slightly in excess of the amount required to maintain the bath substantially free 0 of ferrous iron, said excess, however, not to exceed the equivalent of approximately .03 lb. of nitrous acid per gallon of electrolyte and further wherein the electrode is rubbed before it is immersed in the electrolyte.

5 15. The method of claim 4 wherein the electrode is rubbed before it is immersed in the electroiyte.

16. In the art of coating iron or steel with a CERTIFICATE 05 Patent No. 2,32,1 ,39.

water insoluble metallic phosphate, the method which comprises electrodepositing the coating by using the metal as an electrode in an electrolyte containing zinc, phosphoric acid and water, heating the electrolyte, passing a periodically alternating current through said electrolyte by means of said electrode, and periodically adding to the electrolyte nitrous acid or a salt thereof in an amount suflicient to maintain the electrolyte substantially free of ferrous iron.

17. The method of claim 16 wherein the quantity of nitrous acid or a'salt thereof is slightly in excess of the amount required to maintain the electrolyte substantially free of ferrous iron, said excess, however, not to exceed the equivalent of approximately .03 lb. of nitrous acid per gallon of electrolyte.

18. The method of claim 16 wherein the quantity of nitrous acid or a salt thereof is slightly in excess of the amount required to maintain the electrolyte substantially free of ferrous iron, said excess, however, not to exceed the equivalent of approximately .03'lb. of nitrous acid per gallon of electrolyte, and wherein the electrolyte also contains an addition agent selected from the class which consists of a nitrate of the alkali metals, magnesium, cerium and zinc.

GERALD C. ROMIG.

CORRECTION.

October 11 1958 GERALD 0. Rome. It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows column, line 58, claim Page L second l1,for "pound" read gallon; page 5, first column,

line 5, claim lz, for ".0675" read .675; and that the said/Letters Patent should be read with this correction therein that the some may conform to the record of the case in the Patent Office.

Signed and sealed this 22nd day of November, it. D. 1958.

(Se l).

iionry Van Arsdale Acting Commissioner of i'atenta.

' column, line 58, claim tain the concentration of iron in the electrolyte preferably below approximately 25% by weight but never over 116% by weight and wherein the electrode is rubbed beiore it is immersed in the electrolyte.

13. The method of claim 6 wherein thequantity of nitrous acid or a salt thereof is maintained slightly in excess oi the amount required to maintain the bath substantially free oi? ferrous iron, said excess, however, not to exceed the equivalent of approximately .03 lb. of nitrous acid per gallon of electrolyte and wherein the electrode is rubbed before it is immersed in the electrolyte.

14. The method oi claim 6 wherein the current is a periodically alternating current and the quantity of nitrous acid or a salt thereof is maintained slightly in excess of the amount required to maintain the bath substantially free of ferrous iron, said excess, however, not to exceed the equivalent of approximately .03 lb. of nitrous acid per gallon of electrolyte and further wherein the electrode is rubbed before it is immersed in the electrolyte.

15. The method of claim 4 wherein the electrode is rubbed before it is immersed in the electroiyte.

16. In the art of coating iron or steel with a CERTIFICATE OF Patent No. 2,32,1 ,39.

GERALD C.

water insoluble metallic phosphate, the method which comprises electrodepositing the coating by using the metal as an electrode in an electrolyte containing zinc, phosphoric acid and water, heating the electrolyte, passing a periodically alternating current through said electrolyte by means of said electrode, and periodically adding to the electrolyte nitrous acid or a salt thereof in an amount sufllcient to maintain the electrolyte substantially free of ferrous iron.

17. The method of claim 16 wherein the quantity of nitrous acid or a'salt thereof is slightly in excess of the amount required to maintain the electrolyte substantially free of ferrous iron, said excess, however, not to exceed the equivalent of approximately .03 lb. of nitrous acid per gallon of electrolyte.

18. The method of claim 16 wherein the quantity of nitrous acid or a salt thereof is slightly in excess of the amount required to maintain the electrolyte substantially free of ferrous iron, said excess, however, not to exceed the equivalent of approximately .03'lb. of nitrous acid per gallon of electrolyte, and wherein the electrolyte also contains an addition agent selected from the class which consists of a nitrate of the alkali metals, magnesium, cerium and zinc.

GERALD C. ROMIG.

CORRECTION.

October 11 1958 Rome It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows 11, for "pound" read gallon; page 5, first column,

Page L second line 5, claim lz, for ".0675" read .675; and that the said/Letters Patent should be read with this correction therein that the some may conform to the record of the case in the Patent Office.

Signed and sealed this 22nd day of November, it. D. 1958.

(Se l).

lienry Van Arsdale Acting Commissioner of l'atenta. 

